As our living standard is improving gradually, various electronic consumer products are introduced to meet extensive consumer requirements, and thus promoting the prosperity of various industries directly and driving the growth of related sub-industries indirectly. To further meet the consumer requirements and trends for various functions, portability, operability and appearance, in hope of improving consumer's willingness to buy and brand loyalty, various electronic consumer products tend to be designed thinner and lighter. For example, the market share of color mobile phones with a photographic function and other combined functions grows drastically, and the demand of color LCD panels and camera modules for mobile phones rises accordingly. Color LCD panel industry is divided into the area of color super twisted nematic (CSTN) LCDs and thin-film transistor (TFT) LCDs, and the key components including light emitting diodes (LEDs) and flexible printed circuit (FPC) boards also grow with the high demand for flexibility, 3-D circuit layout and light weight of a miniaturized foldable design of mobile phones. The estimated quantity of flexible printed circuit boards used in a color mobile phone is increased from 3˜4 pieces to 6˜7 pieces, and the design of flexible printed circuit boards tends to follow a high-end small circuit specification. A flexible printed circuit board is made by raw materials including a flexible insulating substrate material and a circuit conductor material (generally copper clad), and the raw materials are divided into resins, copper clads, adhesives, coverlays, and flexible copper cladded laminates (FCCL). Since polyimide (PI) has good expansibility and heat resistance, therefore PI is generally used as a resin material and serves as a middle layer and a substrate in the manufacture of flexible copper substrates and also as a coverlay film.
PI manufacturers can produce different PI films from different PI monomers according to different technologies in three main aspects: formula, manufacture process and processing method, and thus different manufacturers achieve different applications and performance of the materials. Further, the flexible copper substrate is divided into two main types: an adhesive three-layer structure and an adhesiveless two-layer structure, and both adopt different manufacture processes, methods and applications, and thus the properties of the materials are different. In general, the adhesive three-layer structure is usually applied to the production of a large number of flexible printed circuit board products and the adhesiveless two-layer structure is usually applied to the manufacture of high-end flexible printed circuit boards, such as the rigid and flexible printed circuit boards and some of the multi-layer boards. It is believed that the adhesiveless two-layer structure will take over some of the adhesive three-layer flexible copper cladded laminates used for the flexible printed circuit boards with high resolution and good dimensional stability.
Referring to FIG. 1, a schematic view of the relation between the raw materials and the finished goods of a prior art flexible printed circuit board is illustrated. In the manufacture of the flexible printed circuit board 150, an insulating substrate material 100 and a circuit conductor material 110 are used to produce an adhesiveless two-layer flexible copper cladded laminate 130 first, and then a coverlay, a stiffener, and an anti-static layer are used to produce the flexible circuit board 150. On the other hand, an adhesive three-layer flexible copper cladded laminate 140 is made of an insulating substrate material 100, a circuit conductor material 110 and an adhesive 120, and a flexible printed circuit 150 is made of such laminate 140. At present, flexible printed circuit boards are generally used in electronic products, particularly mobile phones and LCDs showing a drastic a growth of using flexible circuit boards in their applications.
Referring to FIG. 2, a top view of a flexible printed circuit board and a cross-sectional view of a bonding head according to a prior art are illustrated. The flexible printed circuit board 2 comprises a first insulating layer 200, an adhesive layer 210, a conductive layer 220 and a second insulating layer; wherein the first insulating layer 200 and the second insulating layer 240 are made of the same material or different materials, and the first insulating layer 200 includes a solder pad area 270 and the second insulating layer 240 includes a bonding area 250, such that a bonding head 260 is in direct contact with the bonding area 250 for soldering the flexible printed circuit board 2 with another flexible printed circuit board. In actual practices, the boding area 250 is usually situated at a position substantially parallel to the solder pad area 270, so that heat energy can be conducted from the bonding area 250 to the adhesive layer 210, conductive layer 220 and solder pad area 270 for bonding. However, it is necessary to increase the temperature of the bonding head 260 for bonding, and the high temperature will burn the first insulating layer 200 and the adjacent adhesive layer 210 black, and thus causing poor bonding quality and appearance of the product, or even deteriorating the materials in the bonding area. For example, a bonding machine sets a temperature for the bonding head for a thermal compression, and the temperature of the bonding head is set to 330° C. for a predetermined time (such as 3 seconds for temperature rise) and then the operating temperature of the bonding machine is set to 470° C. for another predetermined time (such as 3.5 seconds for the bonding), then the solder will be fused to complete the bonding process. However, the first insulating layer 200 and its adjacent adhesive layer 210 will be burned black at the temperature of 470° C., and such phenomenon is particularly severe for lead-free solders because the melting point of lead-free solders is higher than that of lead solders. For example, the melting points of the solders of the Sn—Ag—Cu series and Sn—Cu—Ni series are 227° C. and 217° C. respectively. Compared with the melting point 183° C. of solder of the Sn—Pb series, there is a difference of 34˜44° C. Therefore, it is necessary to increase the temperature of the bonding head 260 for lead-free solders in compliance with the environmental protection and international standard requirements. As a result, the burning effect produced in the bonding area 250 becomes obvious and severe.
Therefore, developing a thermal bonding structure and manufacture process for a flexible printed circuit board to overcome the foregoing shortcomings of the prior arts, improving the burning situation in the bonding area, and further conducting heat energy to the solder so as to lower the bonding temperature and supply less heat energy for saving bonding time and costs are important topics for manufactures and users and demand immediate attentions and feasible solutions. The inventor of the present invention based on years of experience on related research and development of the optoelectronic component industry to invent a thermal bonding structure and manufacture process for flexible printed circuit boards to overcome the foregoing shortcomings.